Conventionally, a structure of a touch panel for detecting an input position has been investigated variously, and a resistive type, an electrostatic capacitance type, a surface acoustic wave type, an infrared type, and the like are used. Some resistive types are referred to as a 4-wire type or a 5-wire type.
The 5-wire type touch panel includes transparent and rectangular resistive layers provided on opposed surfaces of two substrates, and a frame-shaped electrode formed along four sides of one of the resistive layers. Each of the sides of the frame-shaped electrode constitutes first and second electrode portions, which are opposed to each other, and third and fourth electrode portions, which are opposed to each other. Moreover, a run-around circuit for a connection to an external driving circuit is provided around the substrate.
Description will be given to a method of detecting a touch position in the touch panel having such a structure. First of all, a + (plus) voltage is applied to both ends of the first electrode portion and a − (minus) voltage is applied to both ends of the second electrode portion to form an electric potential gradient between the first electrode portion and the second electrode portion. Next, switching is carried out to apply a voltage between the third electrode portion and the fourth electrode portion, thereby forming an electric potential gradient in the same manner as described above. When the two electric potential gradients are changed over in a short time by means of a switch to repetitively perform the operation, an electric potential gradient is alternately formed on the substrate in horizontal and vertical directions. When one of the substrates is pressed (touched), the resistive layers come in contact with each other so that one of the substrates can measure an electric potential formed on the other substrate. By detecting the electric potential at that time, it is possible to obtain a touch position.
In the above touch panel, each electrode portion is generally caused to have a proper resistance in order to prevent an increase in a current flowing into a panel in application of a voltage. When the voltage is applied to the panel, consequently, a distortion of an electric potential distribution is caused by a voltage drop due to a resistance of the electrode portion. A degree of the distortion of the electric potential distribution is increased in the vicinity of each electrode portion. Therefore, as shown in FIG. 14, there is a problem in that a frame-shaped area 101 which is provided on an inside of the panel of each electrode portion 100 adjacently to the electrode portion 100 is hard to be used as a touch input portion, resulting in a decrease in an input region 102 of the touch panel.
Moreover, it is proposed that a plurality of T-shaped split electrodes 110 are disposed to constitute an electrode 111 as shown in FIG. 15 in order to reduce the degree of the distortion of the electric potential distribution (see Patent Document 1). In the above touch panel, however, the T-shaped portion of the split electrode is disposed to be protruded toward the inside of the panel. Therefore, it is hard to use, as a touch input portion, a region in which the T-shaped portion is to be disposed. As a result, a frame-shaped area adjacent to the electrode 111 cannot be used as the touch input portion, and consequently, the input region of the touch panel is decreased.